HEAT AND MASS TRANSFER:
FUNDAMENTALS & APPLICATIONS
6th Edition in SI Units
Yunus A. Çengel, Afshin J. Ghajar
Lecture PPT
Lecture slides by
Mehmet Kanoglu
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
Chapter Number 1
INTRODUCTION AND BASIC CONCEPTS
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Objectives
•
Understand how thermodynamics and heat transfer are related
to each other
•
Distinguish thermal energy from other forms of energy, and
heat transfer from other forms of energy transfer
•
Perform general energy balances as well as surface energy
balances
•
Understand the basic mechanisms of heat transfer, which are
conduction, convection, and radiation, and Fourier's law of
heat conduction, Newton's law of cooling, and the Stefan–
Boltzmann law of radiation
•
Identify the mechanisms of heat transfer that occur
simultaneously in practice
•
Develop an awareness of the cost associated with heat losses
•
Solve various heat transfer problems encountered in practice
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1-1 THERMODYNAMICS AND HEAT TRANSFER
Heat: The form of energy that can be transferred from one
system to another as a result of temperature difference.
Thermodynamics is concerned with the amount of heat
transfer as a system undergoes a process from one equilibrium
state to another.
Heat Transfer deals with the determination of the rates of such
energy transfers as well as variation of temperature.
The transfer of energy as heat is always from the highertemperature medium to the lower-temperature one.
Heat transfer stops when the two mediums reach the same
temperature.
Heat can be transferred in three different modes:
conduction, convection, radiation
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Application Areas of Heat Transfer
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Historical Background
Kinetic theory: Treats molecules as
tiny balls that are in motion and thus
possess kinetic energy.
Heat: The energy associated with the
random motion of atoms and
molecules.
Caloric theory: Heat is a fluidlike
substance called the caloric that is a
massless, colorless, odorless, and
tasteless substance that can be
poured from one body into another
It was only in the middle of the
nineteenth century that we had a true
physical understanding of the nature
of heat.
Careful experiments of the
Englishman James P. Joule published
in 1843 convinced the skeptics that
heat was not a substance after all, and
thus put the caloric theory to rest.
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1-2 ENGINEERING HEAT TRANSFER
Heat transfer equipment such as heat exchangers, boilers, condensers, radiators,
heaters, furnaces, refrigerators, and solar collectors are designed primarily on the
basis of heat transfer analysis.
The heat transfer problems encountered in practice can be considered in two
groups: (1) rating and (2) sizing problems.
The rating problems deal with the determination of the heat transfer rate for an
existing system at a specified temperature difference.
The sizing problems deal with the determination of the size of a system in order to
transfer heat at a specified rate for a specified temperature difference.
An engineering device or process can be studied either experimentally (testing and
taking measurements) or analytically (by analysis or calculations).
The experimental approach has the advantage that we deal with the actual physical
system, and the desired quantity is determined by measurement, within the limits of
experimental error. However, this approach is expensive, timeconsuming, and often
impractical.
The analytical approach (including the numerical approach) has the advantage that it
is fast and inexpensive, but the results obtained are subject to the accuracy of the
assumptions, approximations, and idealizations made in the analysis.
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Modeling in Engineering
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1-3 HEAT AND OTHER FORMS OF ENERGY
•
Energy can exist in numerous forms such as:
✓ thermal
✓ mechanical
✓ kinetic
✓ potential
✓ electrical
✓ magnetic
✓ chemical
✓ nuclear
•
Their sum constitutes the total energy E (or e on a unit mass
basis) of a system.
•
The sum of all microscopic forms of energy is called the internal
energy of a system.
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• Internal energy: May be viewed as the sum of the
kinetic and potential energies of the molecules.
• Sensible heat: The kinetic energy of the molecules.
• Latent heat: The internal energy associated with the
phase of a system.
• Chemical (bond) energy: The internal energy
associated with the atomic bonds in a molecule.
• Nuclear energy: The internal energy associated with
the bonds within the nucleus of the atom itself.
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Internal Energy and Enthalpy
•
In the analysis of systems
that involve fluid flow, we
frequently encounter the
combination of properties u
and Pv.
•
The combination is defined
as enthalpy (h = u + Pv).
•
The term Pv represents the
flow energy of the fluid (also
called the flow work).
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Specific Heats of Gases, Liquids, and
Solids
•
Specific heat: The energy required to raise the
temperature of a unit mass of a substance by
one degree.
•
Two kinds of specific heats:
✓ specific heat at constant volume cv
✓ specific heat at constant pressure cp
•
The specific heats of a substance, in general,
depend on two independent properties such as
temperature and pressure.
•
At low pressures all real gases approach ideal
gas behavior, and therefore their specific heats
depend on temperature only.
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•
Incompressible substance: A
substance whose specific volume (or
density) does not change with
temperature or pressure.
•
The constant-volume and constantpressure specific heats are identical
for incompressible substances.
•
The specific heats of incompressible
substances depend on temperature
only.
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Energy Transfer
Energy can be transferred to or from a given
mass by two mechanisms:
when
is constant:
heat transfer and work.
Heat transfer rate: The amount of heat
transferred per unit time.
Heat flux: The rate of heat transfer per unit
area normal to the direction of heat transfer.
Power: The work
done per unit time.
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1-4 THE FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics (conservation of energy
principle) states that energy can neither be created nor destroyed
during a process; it can only change forms.
The net change (increase or
decrease) in the total energy of
the system during a process is
equal to the difference between
the total energy entering and the
total energy leaving the system
during that process.
The energy balance for any
system undergoing any process
in the rate form
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In heat transfer problems it is convenient to
write a heat balance and to treat the
conversion of nuclear, chemical,
mechanical, and electrical energies into
thermal energy as heat generation.
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Energy Balance for Closed Systems (Fixed Mass)
A closed system consists of a fixed mass.
The total energy E for most systems
encountered in practice consists of the
internal energy U.
This is especially the case for stationary
systems since they don’t involve any
changes in their velocity or elevation during
a process.
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Energy Balance for
Steady-Flow Systems
A large number of engineering devices such as
water heaters and car radiators involve mass flow
in and out of a system, and are modeled as
control volumes.
Most control volumes are analyzed under steady
operating conditions.
The term steady means no change with time at a
specified location.
Mass flow rate: The amount of mass flowing
through a cross section of a flow device per unit
time.
Volume flow rate: The volume of a fluid flowing
through a pipe or duct per unit time.
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Surface Energy Balance
A surface contains no volume or mass,
and thus no energy. Thereore, a surface
can be viewed as a fictitious system
whose energy content remains constant
during a process.
This relation is valid for both steady and
transient conditions, and the surface
energy balance does not involve heat
generation since a surface does not
have a volume.
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1-5 HEAT TRANSFER MECHANISMS
•
•
•
•
•
•
Heat as the form of energy that can be transferred from one system to
another as a result of temperature difference.
A thermodynamic analysis is concerned with the amount of heat transfer
as a system undergoes a process from one equilibrium state to another.
The science that deals with the determination of the rates of such
energy transfers is the heat transfer.
The transfer of energy as heat is always from the higher-temperature
medium to the lower-temperature one, and heat transfer stops when the
two mediums reach the same temperature.
Heat can be transferred in three basic modes:
✓ conduction
✓ convection
✓ radiation
All modes of heat transfer require the existence of a temperature
difference.
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1-6 CONDUCTION
Conduction: The transfer of energy from the more
energetic particles of a substance to the adjacent
less energetic ones as a result of interactions
between the particles.
In gases and liquids, conduction is due to the
collisions and diffusion of the molecules during their
random motion.
In solids, it is due to the combination of vibrations of
the molecules in a lattice and the energy transport
by free electrons.
The rate of heat conduction through a plane layer is
proportional to the temperature difference across
the layer and the heat transfer area, but is inversely
proportional to the thickness of the layer.
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When x → 0
Fourier’s law of
heat conduction
Thermal conductivity, k: A measure of the
ability of a material to conduct heat.
Temperature gradient dT/dx: The slope of
the temperature curve on a T-x diagram.
Heat is conducted in the direction of
decreasing temperature, and the temperature
gradient becomes negative when
temperature decreases with increasing x.
The negative sign in the equation ensures
that heat transfer in the positive x direction is
a positive quantity.
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Thermal
Conductivity
Thermal conductivity: The rate
of heat transfer through a unit
thickness of the material per unit
area per unit temperature
difference.
The thermal conductivity of a
material is a measure of the
ability of the material to conduct
heat.
A high value for thermal
conductivity indicates that the
material is a good heat
conductor, and a low value
indicates that the material is a
poor heat conductor or insulator.
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The range of
thermal
conductivity of
various
materials at
room
temperature.
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The thermal conductivities of gases such
as air vary by a factor of 104 from those
of pure metals such as copper.
Pure crystals and metals have the
highest thermal conductivities, and gases
and insulating materials the lowest.
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The variation of
the thermal
conductivity of
various solids,
liquids, and gases
with temperature.
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Thermal Diffusivity
cp Specific heat, J/kg·°C: Heat capacity per
unit mass
cp Heat capacity, J/m3·°C: Heat capacity
per unit volume
 Thermal diffusivity, m2/s: Represents
how fast heat diffuses through a material
A material that has a high thermal
conductivity or a low heat capacity will
obviously have a large thermal diffusivity.
The larger the thermal diffusivity, the faster
the propagation of heat into the medium.
A small value of thermal diffusivity means
that heat is mostly absorbed by the
material and a small amount of heat is
conducted further.
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1-7 CONVECTION
Convection: The mode of
energy transfer between a solid
surface and the adjacent liquid or
gas that is in motion, and it
involves the combined effects of
conduction and fluid motion.
The faster the fluid motion, the
greater the convection heat
transfer.
In the absence of any bulk fluid
motion, heat transfer between a
solid surface and the adjacent
fluid is by pure conduction.
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Forced convection: If the
fluid is forced to flow over the
surface by external means
such as a fan, pump, or the
wind.
Natural (or free) convection:
If the fluid motion is caused by
buoyancy forces that are
induced by density differences
due to the variation of
temperature in the fluid.
Heat transfer processes that involve change of phase of a fluid
are also considered to be convection because of the fluid motion
induced during the process, such as the rise of the vapor bubbles
during boiling or the fall of the liquid droplets during
condensation.
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Newton’s law of cooling
h
As
Ts
T
convection heat transfer coefficient, W/m2 · °C
the surface area through which convection heat transfer takes place
the surface temperature
the temperature of the fluid sufficiently far from the surface.
The convection heat transfer
coefficient h is not a property
of the fluid.
It is an experimentally
determined parameter
whose value depends on all
the variables influencing
convection such as
- the surface geometry
- the nature of fluid motion
- the properties of the fluid
- the bulk fluid velocity
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1-8 RADIATION
• Radiation: The energy emitted by matter in the form of electromagnetic
waves (or photons) as a result of the changes in the electronic
configurations of the atoms or molecules.
• Unlike conduction and convection, the transfer of heat by radiation does
not require the presence of an intervening medium.
• In fact, heat transfer by radiation is fastest (at the speed of light) and it
suffers no attenuation in a vacuum. This is how the energy of the sun
reaches the earth.
• In heat transfer studies we are interested in thermal radiation, which is
the form of radiation emitted by bodies because of their temperature.
• All bodies at a temperature above absolute zero emit thermal radiation.
• Radiation is a volumetric phenomenon, and all solids, liquids, and
gases emit, absorb, or transmit radiation to varying degrees.
• However, radiation is usually considered to be a surface phenomenon
for solids.
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Stefan–Boltzmann law
 = 5.670  10−8 W/m2 · K4 Stefan–Boltzmann constant
Blackbody: The idealized surface that emits radiation at the maximum rate.
Radiation emitted by real surfaces
Emissivity  : A measure of how closely
a surface approximates a blackbody for
which  = 1 of the surface. 0   1.
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Absorptivity : The fraction of the radiation energy incident on a
surface that is absorbed by the surface. 0   1
A blackbody absorbs the entire radiation incident on it ( = 1).
Kirchhoff’s law: The emissivity and the absorptivity of a surface at
a given temperature and wavelength are equal.
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Net radiation heat transfer: The
difference between the rates of
radiation emitted by the surface and
the radiation absorbed.
The determination of the net rate of
heat transfer by radiation between
two surfaces is a complicated
matter since it depends on
• the properties of the surfaces
• their orientation relative to each
other
• the interaction of the medium
between the surfaces with radiation
When a surface is completely enclosed by
a much larger (or black) surface at
temperature Tsurr separated by a gas
(such as air) that does not intervene with
radiation, the net rate of radiation heat
transfer between these
two surfaces is given by
Radiation is usually significant
relative to conduction or natural
convection, but negligible relative
to forced convection.
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When radiation and convection occur
simultaneously between a surface and a gas:
Combined heat transfer coefficient hcombined
Includes the effects of both convection and radiation
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1-9 SIMULTANEOUS HEAT
TRANSFER MECHANISMS
Heat transfer is only by conduction in opaque solids, but
by conduction and radiation in semitransparent solids.
A solid may involve conduction and radiation but not
convection. A solid may involve convection and/or
radiation on its surfaces exposed to a fluid or other
surfaces.
Heat transfer is by conduction and possibly by radiation
in a still fluid (no bulk fluid motion) and by convection
and radiation in a flowing fluid.
In the absence of radiation, heat transfer through a fluid
is either by conduction or convection, depending on the
presence of any bulk fluid motion.
Convection = Conduction + Fluid motion
Heat transfer through a vacuum is by radiation.
Most gases between two solid surfaces do not
interfere with radiation.
Liquids are usually strong absorbers of radiation.
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1-10 AEROGEL—A REMARKABLE
SUPERINSULATING MATERIAL
Aerogels—often called “frozen smoke”—are
lightweight and have an ethereal blue haze.
An aerogel in your hand feels like a piece of air—but
it is a solid.
What makes them so interesting is their unique
nanoporous structure.
This structure is formed through the sol-gel process,
whereby a sol (a particulate-containing solution)
undergoes a polymerization reaction to form a
nanoporous wet gel.
The key to making an aerogel is to carefully dry the
wet gel.
Supercritical extraction is commonly used to make
aerogels.
Aerogels can be made from many different precursor
materials.
The most common type of aerogel is the silica
aerogel.
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When an aerogel is made by
the sol-gel method, it forms a
nanoporous structure
comprised of clusters of silica
spheres on the order of 5–10
nm that connect and form long,
chain-like connecting structures
with pores that range from 5 to
100 nm in size.
Silica aerogels are very porous:
90–99 percent air
Have low density:
0.05–0.4 g/cm3
Large surface areas:
500–1400 m2/g
Highly translucent and have low
thermal conductivity:
0.01–0.3 W/m⋅K
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Convection through an aerogel is hindered by the small pore size.
At low temperatures radiation is not significant.
The primary method for heat transfer through an aerogel is by conduction in
both the solid and gaseous phases.
Solid phase conduction is related to the thermal conductivity of the solid
and the amount of that solid.
Since aerogels are so porous, there is very little solid material, and what is
there follows a tortuous path with many dead ends (formed by the original
sol-gel process).
For these reasons, the thermal conductivity of an aerogel is very low.
Aerogels are commercially available in an impregnated blanket form, as
granules, or as monolithic pieces.
They are used in a variety of applications, including comet dust collectors,
oil spill clean-up material, and chemical sensors.
The most common application for aerogels is as insulation where, for
example, they are used in daylighting products (insulating windows), highperformance clothing, and pipe insulation.
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Heat loss through a 6-mm -thick single-pane glass window
If we could make the window pane out of a monolithic aerogel
under slight vacuum, the heat loss would be:
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1-11 PREVENTION THROUGH DESIGN
The first of the fundamental canons of ethics for engineers is to “hold
paramount the safety, health, and welfare of the public” when fulfilling their
professional duties.
In 2007, the National Institute for Occupational Safety and Health (NIOSH)
launched the National Prevention through Design (PtD) initiative, with the
mission to prevent or reduce work-related injuries, illnesses, and fatalities by
including prevention considerations in all circumstances that impact
individuals in the work places.
As such, the concept of PtD involves applying the means of reducing risks
and preventing hazards in the design of equipment, tools, processes, and
work facilities.
The concepts of PtD can also be rationally applied to preventing
failures and damages of devices, products, and systems. Since such failures
and damages are often led to negative impacts on the environment,
profitability, and ultimately the society at large.
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Within the context of heat and mass transfer,
the PtD concepts can be presented along with
the physical mechanisms involved and
practical applications.
Issues such as prevention of thermal burn, fire
hazard, and thermal failure in systems are
topics that can relate the concepts of PtD with
the basic science of heat and mass transfer.
The process of solving heat and mass transfer
problems, along with the
application of PtD concepts, involves
incorporating prescribed PtD criteria,
the prevention of burn injury, fire hazard, or
system failure, to the solutions.
To successfully arrive at a solution that
satisfies prescribed PtD criteria requires the
understanding of how the physical
mechanisms of heat and mass transfer
interrelate with the concepts of PtD.
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1-12 ENGINEERING CODES AND STANDARDS
Engineering codes and standards are ubiquitous.
When we consider any given device that we use daily, the path, from the
design to the prototype and finally to the production of the final product is
guided by standards and regulated by codes.
A product that has been manufactured to meet certain standards tells the
user about the product’s quality and that it has met certain levels of scrutiny
set forth by those standards.
In essence, engineering standards communicate to the user that the product
is expected to function safely and meets quality objectives.
At the industrial level, engineering codes and standards provide efficiency in
engineering processes (from design to manufacturing), maintain safety and
uniformity of products that allows interchangeability, and improve commerce.
Engineering codes and standards are evolutionary—they are continually
revised to reflect technical advances, such as new technologies, new
designs, new procedures, and new materials.
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The development of engineering codes and standards is rooted in the
engineers’ code of ethics to “hold paramount the safety, health, and welfare
of the public” in their professions (Code of Ethics for Engineers, 2007).
The founding of the American Society of Mechanical Engineers (ASME) in
1880 provided a forum for engineers, designers, and machine shop
managers to discuss and collaborate to solve issues brought on by the
industrial age.
Within four years, the first ASME standard, “Code for the Conduct of Trials
of Steam Boilers,” was issued.
This was the first ASME Performance Test Code (PTC). Today, there are
close to 50 ASME PTCs, and they cover components, systems, and
facilities.
Performance test codes are used by manufacturers to ensure the reliability
of their products, and users can use the PTCs to protect themselves from
products that function poorly.
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Some of the very first engineering codes and standards were needed
because of engineering failures that caused the loss of life and property.
In 1905, a boiler exploded in a shoe factory in Brockton, Massachusetts. In
the next year, a similar incident happened in another shoe factory in Lynn,
Massachusetts.
Within a decade, the first edition of the Boiler and Pressure Vessel Code
(BPVC) was issued by the ASME in 1914 and published in 1915.
Today, the ASME BPVC has been adopted by the United States and Canada,
and it is being used in over 100 countries around the world.
A recent study done by ASME Vision 2030 Project reveals that almost 50
percent of engineers in their early career are unfamiliar with engineering
codes and standards.
As companies and manufacturers expand operations globally, there is a
greater need to harmonize codes and standards across jurisdictions and
disciplines.
The need for engineers to know about codes and standards is growing, and
having this knowledge allows engineers to innovate and manufacture
competitive products.
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In the subject of heat and mass transfer, there are several engineering codes
and standards that are relevant.
These codes and standards have been issued and published by professional
associations, such as the ASME and the ASHRAE, and standards
organizations, such as ASTM International, the ANSI, and the ISO.
For instance, the 2015 ASME Boiler & Pressure Vessel Code, Section IV
(ASME BPVC.IV-2015) provided the permitted O-ring materials and their
temperature limits for water tube attachments in hot water boilers.
Another example is the standard specification issued by ASTM International
for polyethylene insulation used on electrical cable (ASTM D1351-14, 2014),
which is important as overheating can rapidly deteriorate the insulation.
Extreme temperatures and thermal cycles can cause materials to deteriorate
and thereby affect the functionality of components.
The knowledge of heat and mass transfer, along with adherence to the
relevant codes and standards, allow engineers to analyze, design, and build
components and systems to function within the design conditions.
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1-13 PROBLEM-SOLVING TECHNIQUE
Step 1: Problem Statement
Step 2: Schematic
Step 3: Assumptions and Approximations
Step 4: Physical Laws
Step 5: Properties
Step 6: Calculations
Step 7: Reasoning, Verification, and Discussion
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1-14 ENGINEERING SOFTWARE PACKAGES
All the computing power and the
engineering software packages
available today are just tools, and tools
have meaning only in the hands of
masters.
Hand calculators did not eliminate the
need to teach our children how to add
or subtract, and sophisticated medical
software packages did not take the
place of medical school training.
Neither will engineering software
packages replace the traditional
engineering education. They will simply
cause a shift in emphasis in the courses
from mathematics to physics.
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Equation Solvers
Despite its simplicity, Excel is commonly used in solving systems
of equations in engineering as well as finance. It enables the user
to conduct parametric studies, plot the results, and ask “what if ”
questions. It can also solve simultaneous equations if properly set
up.
Engineering Equation Solver (EES) is a program that solves
systems of linear or nonlinear algebraic or differential equations
numerically.
It has a large library of built-in thermodynamic property functions
as well as mathematical functions.
Unlike some software packages, equation solvers do not solve
engineering problems; they only solve the equations supplied by
the user.
Therefore, the user must understand the problem and formulate it
by applying any relevant physical laws and relations.
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1-15 ACCURACY, PRECISION,
AND SIGNIFICANT DIGITS
Accuracy error (inaccuracy): The value of
one reading minus the true value. In general,
accuracy of a set of measurements refers to
the closeness of the average reading to the
true value. Accuracy is generally associated
with repeatable, fixed errors.
Precision error: The value of one reading
minus the average of readings. In general,
precision of a set of measurements refers to
the fineness of the resolution and the
repeatability of the instrument. Precision is
generally associated with unrepeatable,
random errors.
Significant digits: Digits that are relevant
and meaningful.
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In engineering calculations, the information given
is not known to more than a certain number of
significant digits, usually three digits.
Consequently, the results obtained cannot
possibly be accurate to more significant digits.
Reporting results in more significant digits
implies greater accuracy than exists, and it
should be avoided.
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Summary
• Thermodynamics and Heat Transfer
✓ Application areas of heat transfer
✓ Historical background
• Engineering Heat Transfer
✓ Modeling in engineering
• Heat and Other Forms of Energy
✓ Specific heats of gases, liquids, and solids
✓ Energy transfer
• The First Law of Thermodynamics
✓ Energy balance for closed systems (Fixed Mass)
✓ Energy balance for steady-flow systems
✓ Surface energy balance
• Heat Transfer Mechanisms
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• Conduction
✓ Fourier’s law of heat conduction
✓ Thermal Conductivity
✓ Thermal Diffusivity
• Convection
✓ Newton’s law of cooling
• Radiation
✓ Stefan–Boltzmann law
• Simultaneous Heat Transfer Mechanisms
• Aerogel – A Remarkable Superinsulating Material
• Prevention through Design
• Engineering Codes and Standards
• Problem Solving Technique
• Engineering Software Packages
• Accuracy, Precision and Significant Digits
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